1. Field of the Invention
The present invention relates to an apparatus and method for separating and concentrating crystals from anaerobic or aerobic sludge, separated water generated when sludge is subjected to a concentration process or dewatering process, or various kinds of waste water. The present invention also relates to an apparatus and method for treating digested sludge generated when organic waste matter or waste water is subjected to anaerobic digestion.
2. Description of the Related Art
In a treatment facility for treating organic waste water containing phosphorus and nitrogen, such as sewage, waste water, and screen, first the raw sludge (also referred to as preliminary sludge hereafter) is subjected to liquid/solid separation in a preliminary sedimentation tank, whereupon the separated supernatant liquid is subjected to activated sludge treatment to remove organic matter. During the activated sludge treatment, the multiplied activated sludge is discharged as excess sludge. Incidentally, when raw sludge or organic waste matter such as excess sludge, screen, or raw refuse is subjected to anaerobic digestion, the organic matter in the waste matter is broken down by the action of acidogenic bacteria and methanogenic bacteria, as a result of which the amount of sludge is reduced and gas containing methane, carbon dioxide, and the like is generated together with waste water having high nitrogen and phosphorus concentrations. Nowadays, a great deal of research is being conducted into the utilization of generated methane gas as a heat source, as well as the generation of MAP from digested sludge liquor obtained through the dewatering of digested sludge and the effective use of this MAP in fertilizers, chemical raw materials, and so on (Japanese Unexamined Patent Application Publication 2003-117306).
Moreover, improvements are currently being made in the efficiency of anaerobic digestion tanks in the areas of energy recovery and sludge reduction. For example, by subjecting raw sludge, excess sludge, or mixed sludge containing both raw sludge and excess sludge to physical/mechanical treatment, chemical liquefaction treatment, heat treatment, and so on such that the sludge is solubilized, improvements have been achieved in the methane gas recovery rate and sludge reduction rate during a subsequent anaerobic digestion process. The aforementioned physical/mechanical treatment includes ultrasonic treatment and crushing using a mill. Chemical liquefaction treatment includes treatment using ozone, hydrogen peroxide, acid, and alkali. Heat treatment includes treatment using thermophilic bacteria. For example, Japanese Unexamined Patent Application Publication 2002-336898 describes a method of solubilizing sludge by treating the sludge in an ultrasonic treatment process.
As the efficiency of anaerobic digestion increases in the manner described above, waste water having ever higher nitrogen and phosphorus concentrations is produced. Originally, organic waste matter contains elements such as nitrogen, phosphorus, and also magnesium and the like, and when the organic waste matter is solubilized, these elements migrate into the solution. When waste water containing such high concentrations of nitrogen and phosphorus is returned to a water treatment system, the nitrogen and phosphorus loads on the water treatment system increase, causing deterioration in the quality of the treated water.
Hence, in a known technique, MAP is generated by subjecting the digested sludge or digested sludge liquor to aeration treatment, and a part of the sludge that settles in the sedimentation tank is returned to the aeration tank to serve as seed crystals. In so doing, the phosphorus concentration of the return water decreases, and the phosphorus can be recovered easily as MAP. In Japanese Examined Patent Application Publication H7-115979, digested sludge is decarbonated, whereupon a magnesium compound is added to precipitate MAP, thereby reducing the phosphorus concentration of the dewatered separated liquid. In both cases, the phosphorus concentration of the liquid is reduced by precipitating MAP, and as a result, phosphorus can be prevented from circulating endlessly between water treatment process and sludge treatment process.
Nowadays, in order to achieve effective utilization and efficient treatment of digested sludge, in some regions, all wastewater treatment plants and the like are connected by sewers (pipes), and generated digested sludge is transported through the sewers, gathered at a single treatment plant, and treated collectively in the single treatment plant. Similarly, in order to treat the separated liquid that is generated by dewatering digested sludge efficiently, in some regions, all wastewater treatment plants and the like are connected by sewers, and the generated separated liquid is transported through the sewers to be treated collectively in a single treatment plant. The construction cost of the sewers is cheaper than that of a treatment facility, and since the advantage of scale applies to sludge treatment facilities (i.e. the unit cost thereof decreases as the scale increases), sewers are considered to be more economical in locations such as urban areas, where residential buildings are in close proximity.
However, when digested sludge or the separated liquid thereof is transported through pipes, so-called MAP precipitate is generated when the magnesium ions of the sludge chemically combine with the phosphate ions and ammonium ions, leading to possible blockages of the sludge pipe.
To solve this problem, a method of transporting the sludge through a sludge pipe after removing and recovering the MAP by aerating the digested sludge in a reactor in advance to generate MAP particles, and then subjecting the sludge containing the MAP particles to centrifugal separation, is known. Further, a part or all of the MAP particles is returned to the reactor following centrifugal separation to serve as kernels for generating new MAP particles in the reactor. By performing such an operation, problems such as blockages of the sludge pipes caused by MAP particles can be avoided.
As noted above, as the efficiency of anaerobic digestion increases, waste water having ever higher nitrogen and phosphorus concentrations is produced. Originally, organic waste matter contains elements such as nitrogen, phosphorus, and also magnesium and the like, and when the organic waste matter is solubilized, these elements migrate into the solution. Nitrogen, phosphorus, and magnesium are the constituent components of MAP, and at high concentrations in liquid, or when the pH rises, the nitrogen, phosphorus, and magnesium easily equal or exceed the solubility product of the MAP such that the MAP precipitates spontaneously in a digestion tank. When the MAP precipitates into a draft tube in the digestion tank, the flow of the digested sludge deteriorates, and scale trouble such as pipe blockages during pump extraction occurs frequently. Moreover, the MAP is disposed of together with the dewatered sludge rather than being recovered, and hence there is demand for an efficient MAP recovery method.
When MAP is precipitated by decarbonating the digested sludge or adding a magnesium compound thereto, the phosphorus concentration of the dewatered separated liquid decreases, and therefore the phosphorus load on the water treatment system is reduced so that the treated water can be maintained at a favorable quality. However, this method focuses on phosphorus removal rather than the recovery of phosphorus resources, and hence there is demand for a treatment method which satisfies aspects of both phosphorus removal and phosphorus recovery.
Moreover, when MAP is recovered through aeration and centrifugal separation, the recovered matter contains digested sludge and screen residue as well as the MAP, and hence it is not always possible to recover MAP having a high degree of purity. When recycling phosphorus, purity is required, and hence there is demand for a method of recovering MAP having a high degree of purity.
Furthermore, when MAP is recovered by subjecting the MAP to centrifugal separation using a hydrocyclone and the concentration of MAP or other inorganic solids introduced into the hydrocyclone is high, the hydrocyclone itself may become blocked. In certain cases, the MAP concentration in an overflow riser may also increase, leading to deterioration in the recovery rate. Hence, there is demand for a separating method enabling treatment to be performed with stability and a high MAP recovery rate.
Further, when digested sludge containing MAP is transported by pipe to a facility for treating sludge collectively, a large amount of MAP scale is generated in the pipe, leading to deterioration in the efficiency with which the sludge is transported. Following its initial generation, MAP scale continues to grow. If MAP scale is left on the inside of the pipe, the entire sewer eventually becomes covered in MAP scale, making sludge transportation difficult, and hence cleaning and complicated maintenance must be performed periodically.
Nowadays, due to the increase in sewerage, advancements in advanced treatment, and so on, the amount of sewage-treated sludge is increasing. In order to detoxify and solubilize the sludge, sludge smelting treatment is becoming more widely used. However, when sewage sludge is subjected to smelting treatment at a high temperature between 1200 and 1400° C., a part of the phosphorus contained in the sludge is emitted into the slag without solidifying, and as a result, the phosphorus becomes stuck during an exhaust gas treatment process, leading to problems such as machinery corrosion, an increase in the phosphorus load of the return water produced by exhaust gas wet cleaning, and so on. Hence, there is demand for a technique for removing phosphorus from sludge in advance.
As for crystallization reactors, when attempts have been made in the past to reduce the size of the crystallization reactor, the concentration ability of the hydrocyclone has posed a problem since it becomes difficult to maintain the MAP in the crystallization reactor at a high concentration.